Ok we have ideas but how should they look? 1.insects only 2.insectoids and insects 3.insectoids only -Centipede (weak hp strong attack, can dig underground, fast in underground, easily able to ambush but it need more turn to make) how digging would look like? In my opinion centipedes could have spec...

That is exactly what I mean
About buildings I think
Basic buildings
Nest: ground units trainer
Hive: flying units trainer
Gems buildings
Queen: similar to ent trainer
(can move and train in the same time)
Great colony: can train special units and shoot poison

There is four riders of apocalypse: -Death -War -Pestilence -Famine (I also saw version with conquest but in my opinion this fit better) Look: Death: Rider with sickle and skeleton horse War: Rider with sword and burning horse Pestilence: Rider with bow and rotten horse Famine: Rider with scale and ...

maybe still low speed and small spell range
but
have chances to reanimate stronger units (fire skull thrower armored skeletons)
or
have chances to summon spirits (banshee Shade or something like wraith(new unit))

I think necromancer should be faster I know wagon
but I think they should be faster anyway
he can summon flesh golem but before they reach corpses they can vanish
lich is slow but he has long reanimate range